Power distribution systems include technology to couple sources of power to loads while protecting the distribution infrastructure and maintaining service via circuit protection, fault isolation, circuit reconfiguration (typically for restoration of service to stranded, load-side customers) and system return-to-normal functions. For example, the power distribution system may include circuit switching and fault protection devices including: source protection devices, such as circuit breakers, load protection devices, such as fuses, and fault protection devices, such as fault interrupters, sectionalizers, reclosers and the like, that segment a distribution line and permit fault isolation. While various strategies may be employed to manage the power distribution system to maintain service and to protect the power distribution system, typically the fault protection devices should operate in a coordinated manner to optimize performance of the power distribution system and to minimize the scope and duration of service interruptions. That is, to isolate a fault at the fault protection device nearest the fault to protect the source and to preserve service to loads between the source and the fault protection device.
At the same time, the power distribution system should be manageable, recoverable and operable at a high level of performance with reduced burden. These goals become difficult to obtain as the distribution system becomes heavily populated with distributed, intelligent devices that allow an operator to manage and control the distribution of power and protect the distribution infrastructure.
Wide area communication systems have been employed for several decades as a means to enhance the automation of electric power distribution systems to provide management, improved operation and system recovery. These systems are responsible for controlling the distribution of power from sources/substations out over medium voltage feeders/distribution lines to consumers and are typically radio based due to the high cost of providing fiber or other fixed communication media over a wide geographic area. An example of commercial communication products include the Utilinet radio, sold by Schlumberger, Inc. Most of these products are used in conjunction with SCADA systems, or other low to medium-speed communication applications such as the IntelliTEAM® circuit reconfiguration system, available from S&C Electric Company, Chicago, Ill.
Many aspects of the management and control and particularly the fault protection of the power distribution system, on the other hand, require high speed (low latency) and high reliability communications. Such systems are again preferably radio-based to take advantage of the ease and low cost of installation. An example of such a system includes the HRDS system available from S&C Electric Company. These systems utilize dedicated point-to-point links and dedicated communication channels for each pair of communicating devices. A company called Freewave Communications offers a radio-based off-the-shelf product for use in conjunction with the Schweitzer Engineering Laboratories, Inc. (SEL) mirrored-bits communication protocol. With these two technologies, digital status points can be conveyed between two interconnected distribution automation control devices over radio-based communication infrastructure.
Mesh-topology communication systems, communication systems based upon the Internet's Ad-Hoc Routing methodology, spread-spectrum radio communication systems and, in particular, wireless network communication architecture based upon the IEEE 802.11 standard have found application to provide radio-based communication infrastructure for power distribution systems. The 802.11 standard, in fact, provides a simple and readily implemented solution using off-the-shelf hardware and software.
Security is vitally important to protect the power distribution infrastructure from unauthorized access, reconfiguration or misconfiguration or even terrorist attack. Security in accordance with the IEEE 802.11 standard, for example, comes in two layers. No single element provides an impenetrable protective barrier, so protection is built in layers of methods of operations and particular behaviors.
The IEEE standard provides two basic network architectures: infrastructure and ad hoc. In the infrastructure type network, there is a master station, called an access point (AP) that broadcasts its identity, i.e., service set identifier or SSID, and responds to requests for association. A wireless station that wants to associate with the AP sends a request and will receive back a message indicating that it is now associated with the AP. The AP controls making all associated stations take turns to avoid collisions—two transmitting at once.
In the ad hoc type network there is no master station or access point, per se, just a collection of nearby stations indicating their willingness to participate in an ad hoc network. This is accomplished with the transmission of particular types of network management messages. There is also a distinction made within ad hoc networking, that of attempting to initiate an ad hoc network and that of merely being willing to join an ad hoc network if one should happen to form in the presence of the merely-willing-to-join station.
In the ad hoc network setup process, nothing happens unless at least one station is sending out a message requesting others to participate in an ad hoc network. There could be ten potential participants within range, but no network would form unless at least one station suggested the idea. Suggesting the idea is accomplished via a special management message.
The 802.11 standard also provides that each AP is configured to broadcast a BEACON frame. The periodicity of the BEACON frame may be adjusted, but in each instance the BEACON frame must be provided. Furthermore, the BEACON frame must contain a minimum data set including: timestamp; beacon interval; capability information; SSID; supported rates; one of FH/DS/CF parameters sets, IBSS parameter sets (for ad hoc networks) and TIM for the AP. The SSID is a sort of password that identifies the AP. The SSID may be set to null in the BEACON, in which case the BEACON, while still broadcast by the AP does not identify the AP.
A station wishing to associate with an AP may identify an available AP in one of two ways: actively by sending a PROBE REQUEST or passively by simply listening for the BEACON. If the SSID is set to null, the station can scan the BEACON but cannot identify and associate with the AP because it lacks the SSID. If the AP SSID is known to the station, however, it can send a PROBE REQUEST with the AP SSID to which the AP responds with an acknowledgement message. An association can be established provided that other identification/security authentication/encryption is successful.
As apparent from the standard, an AP either broadcasts its SSID or responds to PROBE REQUESTs containing its SSID, e.g., when the SSID field of the BEACON is set to null. An intruder may learn the AP SSID either from the BEACON or by listening to PROBE REQUESTs. The intruder may then use the learned SSID to initiate its own PROBE REQUEST or use other methods to attempt to gain access to the network via the AP.
What is needed is communication access system or protocol that does not in and of itself render the network vulnerable to unauthorized access. The system and method should do so without requiring complex, time-consuming configuration and preferably using off-the-shelf or only modestly modified off-the-shelf hardware and software.